1. Field
Embodiments described herein relate generally to a nonvolatile semiconductor memory device.
2. Description of the Related Art
Recently, attention has been paid to nonvolatile memories configured as an array of memory cells each including a variable resistance element and connected at an intersection of a word line and a bit line.
Known as nonvolatile memories of this type are a PCRAM (Phase-Change Random Access Memory) using a chalcogenide element as a variable resistance element, a ReRAM (Resistance Random Access Memory) using a transition metal oxide element, a CBRAM for varying a resistance by forming a bridge (a conducting bridge) between electrodes by precipitating metal cations and destroying the bridge by ionizing the precipitated metal, etc. A characteristic of these variable resistance memories is that the memories store changes of the resistance as information.
A PCRAM controls the resistance of a chalcogenide element by changing the phase of the element between a crystallized state and a non-crystallized state by controlling its process from heat generation to cooling by means of the shape, i.e., size, width, etc. of a current/voltage pulse to be applied to the element. ReRAMs include a bipolar type and a unipolar type. The bipolar type controls the resistance of a transition metal oxide element by means of the direction of a current/voltage pulse to be applied to the element. On the other hand, the unipolar type controls the resistance of a transition metal oxide element by means of the size, width, etc. of a current/voltage pulse to be applied to the element.
The unipolar type is preferable for realizing a high-density memory cell array. This is because the memory cell array of the unipolar type is formed without a transistor but with a variable resistance element and a rectifying element such as a diode, etc., which are stacked at each cross-point of a bit line and a word line.
In the case of a unipolar type ReRAM, data is written into the variable resistance memory when a program voltage of, for example, approximately 6.0V is applied to a variable resistance element for approximately 10 ns. This causes the variable resistance element to change from a high resistance state to a low resistance state. This change of states is referred to as being “programmed” or “set”. When an erasing voltage of approximately 2.0V is applied to a data-programmed variable resistance element to make a current of 1 μA to 10 μA flow for 200 ns to 1 μs, the variable resistance element changes from a low resistance state to a high resistance state. This change of states is referred to as being “erased” or “reset”. Stabilization of resetting is an important issue for improving reliability of a nonvolatile memory using such a variable resistance element.